Free Audio Service Manuals - Free download Scott LK 72 Assembly Instructions

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signals of its own. Many ampliﬁers under certain con-
ditions will start to produce a powerful supersonic os-
cillation. This can happen when the ampliﬁer is used
with electrostatic speakers (which act as a capacitive
load), or if the ampliﬁer is operated with no speakers
attached (no load). In extreme cases this oscillation is
capable of burning out an ampliﬁer in a few minutes.
H. H. Scott amplifiers, like the LK-72, are completely
stable under all conditions-no load, resistive, induc-
tive or capacitive load. This means that if the speaker
wire should accidentally be disconnected, you need
have no fear of damaging your ampliﬁer.

We have often referred to the desirability of low
distortion. A wide frequency response, ample power,
and low hum and noise are important but cannot com-
pensate for high distortion. The LK-7Z will produce
its rated output of 40 watts (IHFM standard) at less
than 0.8% total harmonic distortion. This is important,
but even more important is the distortion at normal

- 'Iistening levels. Many ampliﬁers will exhibit reasonably

low distortion at full output, but actually turn out to
have as much or more distortion at normal listening
levels of l to 5 watts. This is self-defeating.

At normal listening levels, the total harmonic distor-

tion of the LK-72 is below 0.05%, which means it is so
lOW it cannot be accurately measured with even the
ﬁnest of laboratory instruments! The translucent sound
from your LK-72 is proof of this low distortion.

Power Supply

In order to supply the necessary voltages to all the
stages of the ampliﬁer a rugged power supply is needed.
The husky oversize power transformer along with the
GZS4/5AR4 rectiﬁer and SRI selenium rectiﬁer com-
prise the heart of the power supply. An important
characteristic of the G234 is its slow warmup. This
gives the other tubes and components an opportunity
to reach normal operating condition before the full
force of the high voltages and currents are transmitted
by the rectiﬁer. The electrolytics (condenser cans)
are all specially designed and conservatively operated.
These are other factors in the long life you can expect
from the ampliﬁer.

Heat can be a problem in any high power ampliﬁer.
Under normal conditions you can expect the ampliﬁer
to use about 200 watts of electricity. This will generate
a considerable amount of heat. The heat has to be
rapidly transferred away from the output stages where
most of it is generated. If it is not, the life of the tubes
and other components will be greatly curtailed. One of
the ways the heat is carried away is by convection to the
air. The many holes in the chassis are designed to
encourage air circulation which well perform this func-
tion. Equally important is the use of electrolytic alumi-
num for the chassis material.

Most ampliﬁer chassis are steel which is a less expen-
sive material. Unfortunately steel is not a good con-
ductor of heat so such units will have their output
stages overheating even though the rest of the ampliﬁer
is cool. Aluminum is an excellent conductor of heat, so
it rapidly conducts the heat away from output stages
to other parts of the chassis where it can be readily
transferred to the air.

In fact you may notice that your aluminum front
panel is warm to the touch. This means that the heat is
being transported away from the heat sources and that
your ampliﬁer is operating safely.

Another advantage of the aluminum chassis is that
it is nonmagnetic so acts as a shield against induced burn
from the power transformer. Conventional chassis made
of steel, which is magnetic, are much more prone to
this type of difﬁculty.

Conclusion

There are many ingredients involved in designing a
high quality complete stereo ampliﬁer. However, as
this discussion has attempted to show, compromise is
not one of them.

Scott LK 72 Assembly Instructions

Extracted text from Scott LK 72 Assembly Instructions (Ocr-read)

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